This invention relates to water-in-oil high internal phase emulsions and the porous polymeric materials produced therefrom.
Water-in-oil emulsions are dispersions of discontinuous or discrete water particles commonly referred to as the "internal" aqueous phase in a continuous or "external" oil phase. Emulsions can contain as much and more than 70 volume percent internal phase. These are often referred to as high internal phase emulsions (HIPEs). The volume fraction of the internal aqueous phase in such emulsions can be as high as 90 percent and frequently is as high as 95 percent with some HIPEs being reported as high as 98 percent aqueous phase.
The use of high internal phase emulsions (HIPEs) in forming porous polymeric materials are well known and are described, for example, in Shell Oil Company (Shell) U.S. Pat. Nos. 5,210,104 and 5,200,433; Lever Brothers Company (Lever) U.S. Pat. Nos. 4,536,521 and 4,788,225; and The Procter & Gamble Company (P & G) U.S. Pat. Nos. 5,147,345; 5,331,015; 5,260,345; 5,268,224 and 5,318,554. In the described HIPEs, the external oil phase typically comprises a vinyl polymerizable monomer, such as 2-ethylhexyl acrylate and styrene, and a cross-linking monomer such as divinylbenzene. The internal aqueous phase typically comprises water, a radical initiator (if not in the oil phase) and an electrolyte. To form a stable emulsion, a surfactant is added to the oil phase prior to emulsification. Commonly used emulsion stabilizing surfactants include, for example, nonionic surfactants, such as sorbitan esters (e.g., sorbitan monooleate and sorbitan monolaurate). The resulting emulsion is then subjected to polymerization conditions which are sufficient to polymerize the monomers in the oil phase to form a porous polymer.
HIPE polymerization has gained increasing interest, since polymeric foams having the capacity to absorb relatively high amounts of water and other liquids can be produced. Unfortunately, in many applications, e.g., as absorbents, the emulsion stabilizing surfactant should be removed from the porous polymeric foam prior to use. For example, as described in U.S. Pat. No. 5,331,015, the surfactant may be an extractable residue which can be removed through post-polymerization rinses. If not removed, the surfactant residue may create a problem when it comes in contact with sensitive human skin.
However, upon removal of the emulsion stabilizing surfactant, the foam generally becomes less hydrophilic. It must then be further treated to render it sufficiently hydrophilic for use as an aqueous fluid absorbent. One suggested method for rendering the foam more hydrophilic involves chemically modifying (i.e., sulfonating or esterifying) the polymeric foam; see, for example, U.S. Pat. No. 4,536,521. In another method, a hydrophilizing component (i.e., salt and/or a residual emulsifier) can be added to the foam; see, for example, U.S. Pat. No. 5,260,345. However, these techniques involve additional costs and/or are often of limited utility.
It would be desirable to provide a water-in-oil HIPE which, when polymerized, would produce a HIPE foam which does not contain a residual surfactant and which does not require a post-polymerization treatment to render it sufficiently hydrophilic to be useful as an absorbent for hydrophilic liquids.